1. Field of Invention
The field of the currently claimed embodiments of this invention relates to nuclear magnetic resonance systems and methods, and more particularly to systems and methods for measuring nuclear magnetic resonance spin-lattice relaxation time T1 and spin-spin relaxation time T2.
2. Discussion of Related Art
The term adiabatic as applied to nuclear magnetic resonance (NMR) excitation pulses refers to frequency and/or amplitude modulated pulses whose effective B1-field in the frame-of-reference rotating at the Larmor frequency change sufficiently slowly such that the nuclear magnetization M is able to follow it without inducing transitions[1, 2]. Adiabatic pulses are highly-valued for their insensitivity to radio frequency (RF) and static magnetic field (B0) inhomogeneity over ranges determined by the pulse duration τ, flip-angle θ, and transverse RF field (B1) amplitude and frequency sweep [3]. The duration of the pulses is supposed to be shorter than any relaxation processes—whence the term, adiabatic fast passage, originally used to describe these pulses.
In the classic experiment, the frequency was swept linearly through resonance [1, 2]. Nowadays, adiabatic full-passage (AFP; θ=180° and half passage (AHP; θ=90° pulses with B1(t) amplitude/frequency-sweeps that vary as sin/cos, tan/tan h and sech/tan h, offer far superior B1-performance [2-5]. The BIR-4 (B1-insensitive rotation) pulse, which combines four AHP segments, has further extended adiabaticity to flip-angles that can be arbitrarily pre-set anywhere in the range |θ|≦180° [6]. The BIR-4 flip-angle is set by means of two phase-jumps between the segments, which can be phase-cycled to improve accuracy [7].
All of these pulses, and especially the BIR-4 pulses, are intrinsically longer than conventional hard pulses. To the extent that the magnetization M evolves in the transverse plane during the pulse, it is subject to transverse T2 (spin-spin) relaxation decay, even when the pulses are self-refocusing [3, 8, 9]. This dependence is potentially exploitable for measuring T2, or for enhancing T2 contrast. To date, except for the use of spectral linewidths, T2 has been measured with NMR spin-echoes (SEs). The most accurate T2 measurements are derived from the Carr-Purcell-Meiboom-Gill (CPMG) technique [10]. SEs are routinely used to provide critically important T2-dependent contrast and T2 measurements in clinical diagnostic magnetic resonance imaging (MRI) [11].
Conventional MRI of relaxation times is done using the partial saturation (PS) or inversion recovery (IR) methods for T1, and the SE method for T2. In addition, B1 field mapping is often required to correct for inhomogeneity in the RF magnetic field, B1. Also, differences in T2 provide an important contrast mechanism in MRI. T2 is conventionally measured by the spin-echo (SE) method in NMR, which is combined with spatial localization for MRI to obtain T2 images and/or images with T2-enhanced contrast or “T2-weighting”. All of these measurements require multiple acquisitions. Therefore, there remains a need for improved NMR systems and methods.